Sustainable Framework of ChitosanâBenzoxazine with Mutual

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Sustainable Framework of Chitosan-Benzoxazine with Mutual Benefits: Low Curing Temperature, and Improved Thermal and Mechanical Properties Monisha Monisha, Nisha Yadav, and Bimlesh Lochab ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06515 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 28, 2019

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ACS Sustainable Chemistry & Engineering

Sustainable

Framework

of

Chitosan-

Benzoxazine with Mutual Benefits: Low Curing Temperature, and Improved Thermal and Mechanical Properties Monisha Monisha, Nisha Yadav, and Bimlesh Lochab*

Materials Chemistry Laboratory, Department of Chemistry, School of Natural Sciences, Shiv Nadar University, Gautam Buddha Nagar, Uttar Pradesh 201314, India.

*[email protected]

ABSTRACT: Polybenzoxazines (PBzs) are emerging as a highly promising and superior class of thermoset polymers for variety of applications. However, it remains a significant challenge to substantially lower the ring-opening polymerization temperature with an ease in processability. On other hand, biomacromolecule chitosan (CS) is explored extensively, but their practical applications have been precluded by the poor thermal and mechanical properties. Here, we developed a fully biobased copolymer of vanillin benzoxazine (V-fa) monomer with CS, which is effective in providing mutual benefits, effective lowering in ROP temperature of benzoxazine (70 oC) and an enhanced thermal stability of CS (by 85 oC, and with char yield of

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~32%). To understand this unusual lowering in ROP temperature, we investigated structural interaction mechanism between solvated CS and V-fa using in situ NMR studies. The analysis of fully intercalated co-structure demonstrated that there is a strong preference for ROP over Schiff base reaction. It is anticipated that benzoxazine molecules move within the inter-planar distance of CS as supported by powder X-ray diffraction studies. An increase in V-fa content in feed ratio led to a placement of V-fa units from random to a systematic and hierarchical arrangement within the CS framework followed by its subsequent polymerization. The synergistic interactions were further supported by FTIR, DSC, SEM, TGA and tensile studies. Current work represents preparation of CS benzoxazine copolymers using a low-cost, efficient, and sustainable approach to assist metal-free ROP reaction of Bz to afford low curable temperature processable films. A new strategy is devised for the utility of CS-PBzs copolymers enabling their extension to innovative applications in cross domains.

KEYWORDS: Chitosan, Vanillin, Benzoxazine, Biobased, Sustainable, Copolymers

INTRODUCTION In recent years, natural, abundant, and waste resources are opted as alternatives to synthetic polymers either as source for chemical feedstock of starting materials or as polymer to

provide

the

benefits

of

sustainability,

biocompatibility,

eco-friendliness,

and

biodegradation.1-3A complete replacement of synthetic polymers with natural polymers is desired but so far insignificant success is achieved due to their inherent drawbacks of inferior thermal and mechanical properties. Alternatively, such disadvantages can be alleviated by blending bio-polymers with synthetic polymers or copolymerization to acquire better set of properties with novel applications.4 Polybenzoxazines (PBzs) emerged as a promising class of thermoset polymers exhibiting versatility in a wide range of applications. including adhesives,5 flame retardant additives,6 cathodic material in batteries,7, 8 coatings,9, 10 aerospace industries, carbon dioxide adsorbent11 and electronics12. Besides effective compatibilization with many polymers,. they also offer notable properties such as good mechanical strength, high thermal stability, flame 2 ACS Paragon Plus Environment

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retardance, good chemical and electrical resistance, near-zero shrinkage during polymerization, low dielectric properties, and good chemical reactivity.13-19 The polymerization occurs via initiator- or catalyst-free thermally induced ring-opening polymerization (ROP) reaction of 1,3-benzoxazine (Bz) monomer. However, the requirement of a higher temperature to mediate ROP (~ 180 - 280 oC) and brittle nature of cured product affects their industrial processability. To mitigate these disadvantages, Bz monomers are structurally modified with additional functionality,20 or alloyed with other polymers.21 The lowering of cure temperature was mediated by addition of acids22-27 metal salts,25,

28-31

metal organic frameworks,32

nanoparticles,5, 33-34 amines, hydroxides, ammonium salts and their counter ions.35-38 Alternatively, instead of physical blending, Bz units can also be introduced into other polymeric chains either by (i) chemical transformation of its pre-existing pendant amine or phenol groups in the polymer or (ii) covalent tethering of Bz monomer utilizing additional and compatible reactive functionality other than benzoxazine ring.39 Natural polymers are gaining significant interest including chitosan (CS) and chitin. 40, 41

The attractive features of CS includes existence of inherent reactive functional groups

(acetamide, primary amine and alcoholic), low cost (ca. 15-20 $/kg),42 and environmental friendliness. It is produced by deacetylation of chitin, the largest second most abundant biomacromolecule,43 obtained from crab and shrimp shells. Besides advancement of CS in several applications,44-51 their inferior thermal and mechanical properties is one of the major limitation.52,

53

The potential of PBz in CS is relatively less explored. Omura et al.54

demonstrated synthetic possibility of introduction of dangling Bz moieties in CS using primary amine groups in its framework. Alhwaige et al.43 reported the co-reaction of main-chain-type benzoxazine polymer (MCBPs) with CS and the blended material exhibited an improvement in char yield of neat PBz with a simultaneous enhancement in tensile strength of CS. The MCBPs has an inherent both thermoplastic and thermoset properties55, 56 which found to assist their processability with CS. The improvement in properties of Bz monomer with CS, mainly to tackle lowering the temperature of ROP of benzoxazine along with an ease in processability needs to be explored. It has been reported that ammonium ions and counter ions also catalyze ROP of Bz depending on the ionization ability and solubility of the salt.37,

38

We also

anticipitated that amine groups of CS may also assist ROP of Bz in a similar fashion. To understand in detail, we have performed in situ 1H-NMR to study the interaction of functionalities present in CS with Bz in ROP reaction. Previous work suggested, vanillin, a renewable phenol, contains inherent aldehyde functionality, which is attractive to perform Schiff base chemistry.57,

58

Ishida et al. reported a Schiff base of vanillin aniline (V-a) 3 ACS Paragon Plus Environment


benzoxazine monomer with Jeffamine and demonstrated it as a surfactant in emulsification of polystyrene.59 In the present work, we incorporated Bz monomer into CS and studied the subsequent changes and affect on the properties. A fully bio-origin Bz monomer, V-fa, based on naturally occurring raw materials,60-62 vanillin (V) and furfurylamine (fa) was synthesized via solventless condensation reaction. Furthermore, incorporation of furan ring in monomer is expected to improve the performance of copolymers based on previous reports.18, 19, 36 V-fa was chemically incorporated into CS at different loadings in aqueous reaction conditions to maximize the renewable content and to enhance the sustainability aspect. The synthesis of V-fa monomer and its co-reaction with CS was monitored by fourier transform infrared (FTIR), and nuclear magnetic resonance [1H-, 13C- and 1H-13C HSQC (heteronuclear single quantum correlation) NMR] spectroscopy to determine the associated structural changes and to develop mechanistic insight of the reaction. The processability of the copolymer into thin films was achieved at a milder temperature (50 oC). The effect of incorporation of V-fa in CS was studied by differential scanning calorimetry (DSC), thermogravimetry analysis (TGA), and powder X-ray diffraction (XRD). The copolymer fims were analyzed by scanning electron microscopy (SEM) and tensile properties to realize the potential of in situ synthesized PBz network within CS backbone. RESULTS AND DISCUSSION Benzoxazine monomer, V-fa was synthesized via Mannich-like condensation from renewable organic sources vanillin (V), furfurylamine (fa) and formaldehyde under solventless condition, Scheme 1. The crude reaction mixture was purified by simplistic methodology of recrystallization using a greener solvent, ethanol to yield V-fa in good yields (80%). Similar to other benzoxazine monomers, thermally mediated ROP reaction of V-fa proceeds via the cleavage of O-CH2-N in benzoxazine ring to form an iminium ion intermediate, which undergoes crosslinking reaction. The o- and p-position in V-fa are occupied by the methoxy and aldehyde functionalities, respectively restricting their relative unavailability for crosslinking reactions at mild temperatures.63 At higher temperature, >180 oC, aldehyde group in V-fa undergoes oxidative reaction followed by decarboxylation to extend crosslinking reaction at p-position.64 The furan ring in V-fa also undergoes electrophilic aromatic substitution reaction with generated iminium ion to provide additional polymerization sites, Scheme 1.65, 66 The so-formed network of PV-fa is primarily assumed to be linked via Mannich bridges.


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O

OH O

+

O

H2N

OH

O

2 CH2O

ROP

o

80 C, 3 h

O

Furfurylamine (fa)

CHO

Monomer (V-fa) O

OH N

N

Carbenium ion

O

O

O O

O

N CH2

CHO

CHO

Vanillin (V)

O

N

HO O

CHO

CHO

OH

O

N

CHO

O

Crosslinking reaction

O

N CH2 CHO

Iminium ion

PV-fa (Polymer)

Intermediate

Scheme 1. Synthesis and polymerization reaction of V-fa to form PV-fa. Structure and purity of V-fa monomer. The structure of V-fa monomer was confirmed by FTIR, 1H-NMR and 13C-NMR spectroscopy. FTIR spectrum of V-fa showed absence of N-H stretches (~ 3400 - 3300 cm-1) due to amine group in furfurylamine and O-H stretch (~ 3332 cm-1) due to vanillin suggested their complete removal from the monomer (Figure S3). The characteristic absorption band due to benzoxazine ring observed at 1226 cm-1 (C-O-C, asymmetric stretch), 1011 cm-1 (C-O-C symmetric stretch), and 949 cm-1 (oxazine skeletal vibration and C-H out-of-plane bending)67 suggesting successful formation of V-fa.



Figure 1. Structural characterization of V-fa: a) 1H- b) 13C-NMR in CDCl3. 1H-NMR

and

13C-NMR

spectrum of V-fa (Figure 1a-b) showed characteristic proton and

carbon resonances at 4.07, 5.07 and 9.80 ppm, and 48.9, 83.4 and 190.9 ppm due to Ar-CH2N, O-CH2-N and -CHO due to benzoxazine ring and aldehyde group respectively. The absence of signals in the region 2.5 - 4.5 ppm in 1H-NMR (inset, Figure 1a) confirmed no oligomeric impurities in V-fa. The traces of starting raw materials, phenol and amine, and oligomeric impurities in Bz monomer are known to catalyze ROP reaction. The purity of the monomer was further confirmed by HPLC and ninhydrin assay (Figure S4). 6 ACS Paragon Plus Environment

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The fabrication of V-fa into thin films was not achieved despite various trials of melt and solvent-assisted processing conditions. This demands exploration of different processing strategy. It is well known that in addition to covalent crosslinks, PBzs exhibits significant interand intra-molecular hydrogen bonding interactions between ring-opened phenolic -OH, nitrogen atom of Mannich base bridges and polar aldehyde groups. This suggested us to utilize another bio-sourced polymer, CS, which has the capability to interact with both V-fa and PVfa via its pendant free amine and hydroxyl groups present in the backbone. Furthermore, besides providing extensive bridging via H-bonding framework, such groups may also react with V-fa forming covalent bridges. However, efficient packing of CS chains due to intrinsic inter- and intra-molecular H-bonding between primary amine, hydroxyl and acetamido groups demands prior disruption to favour their co-interactions with benzoxazine structures. Earlier reports confirmed amine and hydroxyl groups are effective in catalyzing ROP reaction of Bz.35, 37, 38, 68

This suggested CS would exhibit catalytic effect, besides usual condensation reaction

of amine groups in CS with aldehyde group in V-fa to form Schiff base. This multifaceted benefit of introduction of CS along with being sourced from waste-origin enables it as a promising co-reactant in Bz chemistry. Bz monomer, V-fa was added to a constant concentration of CS at different loadings to form copolymers by simple mixing and resultant mixture was obtained as a clear solution (Figure 2a). Details of the synthesis and experimental methods performed are provided in Supporting information. The copolymers are abbreviated as CVx, where x is denoted as the amount of Vfa in the blend. The CS:V-fa feed ratio is varied as 100:0, 100:5, 100:10, 100:50, 100:70, 100:100 and 100:200 and resultant samples are abbreviated as CS, CV0.05, CV0.1, CV0.5, CV0.7, CV1 and CV2, respectively. With an increase in V-fa content in the blend ratio, a slight yellow to orange colouration is observed. The so-formed films of the respective solution are found to be flexible and showed a drastic reddish colour change (Figure 2b), which is accounted to the ROP of V-fa. Notably, the temperature used for homogenizing (70 oC) and formation of thin films (50 oC) are significantly lower than the reported ROP temperature of V-fa (Ti = 179 and Tp = 205 oC).64 Surprisingly, such extreme lowering of ROP temperature (by  = 109 oC) of V-fa is not reported so far. On contrary to the above copolymerization, under similar reaction conditions, pristine V-fa (as a control) showed no change in colour of solution (Figure 2a), and further upon drying resulted in a non-film forming granular solid. An increase in V-fa content in the feed ratio of the reaction mixture, led to an enhancement in the viscosity of the reaction



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mixture. CV1 showed initiation of gelation behaviour and CV2 exhibited complete gelation along with the presence of unreacted V-fa in the reaction mixture (Figure 2c).

Figure 2. Digital image of CS/V-fa copolymers at different ratios of V-fa prepared under same reaction conditions: (a) reaction mixture; and b) thin films; c) gel formation at CV2 composition with remnant of unreacted V-fa. Mechanistic insights of co-reaction of CS/V-fa. To develop mechanistic understanding of constitutional incorporation of V-fa in CS via covalent and physical interaction better, a systematic analysis by FTIR, NMR, and XRD techniques is crucial. The interaction of CS with V-fa is expected to occur via three modes namely, a) H-bonding, b) ring-opening reaction, and c) Schiff base formation. FTIR analysis of CS/V-fa copolymers. To begin with, synthesis of structural mimic of Schiff base functionality due to reaction of CS with V-fa was performed. Therefore, a control reaction of vanillin with furfurylamine and aniline was performed to synthesize the corresponding aliphatic and aromatic Schiff base (Figure S5) i.e. Vf-, and Va-Schiff base, respectively. The formation of Schiff base was confirmed by FTIR (Figure S6), and 1H- and

13C-NMR

spectroscopy (Figure S7 - S8) with the appearance of CH=N stretch at 1640 and 1620 cm-1 (absence of CHO stretch of vanillin at 1665 cm-1) and 1H- and 13C-NMR characteristic signals at 8.27, 8.34 and 162.3, 160.3 ppm, respectively. There is a significant variation in characteristic of -CH=N- resonance positions as confirmed by both FTIR and NMR spectra, which is associated due to the existence of extended conjugation in Va-Schiff base. 8 ACS Paragon Plus Environment

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The effect of addition of Bz monomer, V-fa to CS was monitored and analyzed by FTIR spectroscopy, Figure 3. The spectra of thin films of copolymers clearly showed noticeable differences as compared to CS and V-fa. The intensity of characteristic peak due to free -NH2 (N-H bending) at 1589 cm-1 in CS remain nearly unaffected despite its involvement in condensation reaction with -CHO group of V-fa suggesting their inaccessibility due to polymeric nature, presence of excessive amino groups and incomplete reaction due to its reversible nature. Therefore, 1589 cm-1 was selected as an internal standard and all the spectra were normalized at this wavenumber. The variation in normalized absorbance intensity of characteristic peaks, Figure 3a-b, indicated a concentration dependent effect of V-fa content in the CS/V-fa copolymers. In the copolymers, the characteristic peak of Bz at 1226 cm-1 nearly vanished at lower ratios suggesting effectiveness of CS in ROP of V-fa. In addition, lower CV ratios exhibited a substantial reduction in aldehyde peak intensity at 1684 cm-1. Simultaneously, a new peak at ~1641 cm-1 (cf. similar to Vf-Schiff base) is evident which is assigned to the C=N stretch of the Schiff base with nearly disappearance of aldehyde peak in V-fa.69 In comparison to V-fa, an appearance of new broad band at ~ 3443 cm-1 in copolymers is further supporting the ROP reaction of V-fa. The resultant polar phenolic O-H and amino groups so-formed due to ring-opening reaction of Bz monomer exhibited extensive inter- and intra-molecular H-bonding interactions with ring-opened Bz structure and amino, acetamido, and hydroxyl groups in CS (Figure 3b). The relative maximum percentage of ROP, aldehyde consumption, and Schiff base formation was found to be ~ 83%, ~ 94%, and ~ 50% respectively, Figure 3c. The formation of Schiff base in acidic aqueous condition is highly reversible due to faster exchange and hydrolysis reaction. The yield of imine formed by reaction of CS with vanillin is reported to be as low as ≤12%.69 The higher % of Schiff base formation in our case, is attributed to the favourable viscosity and solubility effect minimizing the reversibility of the reaction, which is in accordance to literature.70 Furthermore, at very high loadings of V-fa i.e. in CV2, the % ROP was found to decrease to 64% with the retention of aldehyde characteristic peaks (1684, 2840, 2750 cm-1) supporting incomplete conversion of aldehyde group into Schiff base and presence of unreacted V-fa (Figure S9). This can also be visualized due to formation of viscous gel, thereby restricting V-fa mobility preventing its homogenization in the mixture as confirmed by presence of white particulates of unreacted Vfa. However, at higher loading, the solubility of V-fa was found to decrease in the blend (> CV1, with V-fa concentartion as 16.66 mg/mL), (Figure 2c) which mediates its precipitating out of the aqueous reaction mixture due to its hydrophobic nature, as reflected in CV2 mixture. In general, the percentage decrease in -CHO peak in CV2 is not in accordance to its 9 ACS Paragon Plus Environment


consumption of Schiff base formation which can be accounted to its involvement in other chemical and physical interactions which needs further analysis.

Figure 3. FTIR analysis of thin films of copolymers: Normalized stacked FTIR absorbance spectra of CV copolymers in the region a) 2000 - 600 cm-1, and b) 3800 - 2600 cm-1; c) relative % conversion of V-fa monomer to PV-fa polymer, aldehyde consumption and Schiff base formation NMR spectroscopy. Benzoxazine ring in the monomer undergoes cleavage reaction at O-CH2N bond to form polymer structure with mainly true Mannich (-CH2-NR-CH2-) and N, O-acetal linkages, Figure 4. If the o-position of the phenol moiety is blocked or sterically hindered the formation of Mannich linkages is prevented.71 However, ROP proceeded to form thermally labile N, O-acetal structure which latter rearranges at high temperature to form thermally stable phenolic architectures.72 In addition, the formation of N, O-acetal linkages is further favoured at low temperature conditions.30 Therefore, it is anticipated in case of V-fa polymerization to form PV-fa, the major linkages are N, O-acetal along with furan linked general Mannich structures as vanillin possess blocked o- and p-positions, and both reaction and processing are pursued at a relatively very low temperature (70 oC and 50 oC) conditions.

Figure 4. Probable linkages in PBz due to ROP of benzoxazine monomer.


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In order to understand the nature of interaction during copolymerization, NMR spectrum of V-fa, CS and their respective mixtures was monitored under the same volume ratios and concentration as the reaction conditions. V-fa sample was re-recorded to ascribe the resonance positions affected by the change in solvent medium from CDCl3 to D2O-CD3COODCH3COCH3 using 1H-, 13C- and 1H-13C HSQC NMR spectra, Figure S10a-c, respectively. A significant change in proton and carbon resonances was observed by varying the nature of deuterated solvent.73 The signals at 4.21, 4.70 and 9.61 ppm in 1H-NMR spectrum and 45.9, 82.5, 194.7 ppm in

13C-NMR

spectrum are assigned to N-CH2-Ar, O-CH2-N and -CHO,

respectively. CS showed respective broad singlet signals at 1.98 ppm, 3.09 ppm and a multiplet in the range 3.50 - 3.82 ppm for acetamide, H2 (proton adjacent to -NH2 group) and H3 - H6 connected to the non-anomeric C3 - C6 carbons, respectively, Figure S11. It must be noted here, as compared to the reaction in a flask, current reaction was carried out in NMR tube at 70 oC without significant stirring of reaction mixture. A simultaneous colour change from colourless to yellow with concurrent increase in the viscosity of the reaction mixture was observed as noted previously. Apparently, attainment of higher viscosity and no stirring of reaction mixture accounted to a slower kinetics of reaction in NMR tube. For 100% conversion of V-fa to form the Schiff base in CS:V-fa, a ~ 1:2 theoretical ratio of -NH2: -CHO groups is required. The reaction of CS with V-fa was monitored at two different feed ratios of C:V, 1:0.5 and 1:2.3 to form CV0.5 and CV2.3 respectively to understand the quantitative effect of V-fa content in the reaction. The latter ratio i.e. CV2.3 contains an excessive quantity of V-fa and was chosen to drive the equilibrium of formation of Schiff base (-CH=N-) in forward direction and to allow their subsequent detection by solution-based NMR studies. Surprisingly, as compared to C:V (1:2.3), 1H-NMR of C:V (1:0.5) at 30 min, showed an ambiguous downfield shift of signals, Figure S12a, along with disappearance of O-CH2-N signal at 4.70 ppm suggesting either a faster ROP of V-fa or obscuring of O-CH2-N signal by general broadening of D2O signal (in our case). The former possibility was ruled out as 13C-NMR spectrum (Figure S12b) of CV0.5 showed the signal due to O-CH2-N at 82.5 ppm. On the contrary, at a higher ratio of V-fa i.e. in C:V (1:2.3), the O-CH2-N proton and carbon signal persists, which is probably accounted to a higher loading of V-fa that may allow ease to monitor structural changes in V-fa. Therefore, the progress of the reaction was monitored at C:V (1:2.3) by 1HNMR spectroscopy (Figure 5a-b). Initially within 30 min, an insignificant reduction in intensity of O-CH2-N signal was observed. However, the O-CH2-N signal vanished completely within 2 days, confirming pivotal role of CS in ROP of Bz. Notably, Ar-CH2-N signal remain unaffected. The stacked 13C-NMR spectra, Figure S13 of C:V (1:2.3) at different time intervals 11 ACS Paragon Plus Environment


also supported the ring-opening reaction and cleavage of O-CH2-N in V-fa monomer due to CS, in accordance to 1H-NMR studies. Within 1-2 days, smaller intensity signals between 4.3 to 4.4 ppm was observed which is assigned to N, O-acetal linkages. The existence of N, Oacetal linkages are more probable in PV-fa network, which is in accordance to earlier expectations as favoured by structural and reaction conditions limitations. The development of N, O-acetal linkages signals at 4.3 to 4.4 ppm are also observed in V-fa devoid of CS (Figure S15) further supporting development of similar linkages in CV2.3. Furthermore, new signals in the region 2.7 - 4.4 ppm are also apparent, which are associated with general Mannich and phenolic bridges formed due to attack of furan ring on the carbenium ion. The characteristic 1H-NMR

signals due to furanyl group at ~7.5 and 6.5 - 6.4 ppm (Figure S14) recorded at

different time intervals, also revealed significant variations, confirming their involvement in ROP (in case of CV2.3). Additionally,  ppm between Ar-CH2-N and fa-CH2-N signal was increased significantly in 2 days from 11 Hz to 16 Hz (Figure 5a, cf. only ~ 0.2 Hz in V-fa, Figure S15), accounted to the interaction of CS with Bz ring affecting its magnetic environment. However, with the advancement of time to 5 days, a new equal intensity signal at 4.21 ppm is observed which is assigned to the formation CS-N-CH2-N linkage via nucleophilic attack of amino group of CS-NH2 on the carbenium ion intermediate (Figure 5a).74 No such signal is observed in V-fa (Figure S14) further confirming the co-reaction of CS to mediate ring-opening reaction. Such intermolecular reactions between V-fa monomer itself and/or with CS are favoured due to enhancement in viscosity of the mixture. This led to a higher localized concentration of V-fa trapped in CS framework and accounting to an increased susceptibility of CS to react with V-fa. The signal position of azomethine hydrogen of Schiff base is dependent on the nature of aromatic vs aliphatic amine. In general, Schiff bases showed a signal between 8 - 9 ppm varying with the extent of conjugation of imine linkage. This observation is further supported by the synthesized functional mimics of Schiff base of aniline vs furan, which showed CH=N signal at 8.27 vs. 8.34 ppm associated due to lower aromaticity of furan than benzene (Figure S7-8). The aliphatic nature of chitosan may induce further upfield shift CH=N signal to ~ 8 ppm. This supports the assignment of formation of Schiff base between CS-NH2 and V-fa by the appearance of signal due to the azomethine hydrogen at ~8.15 ppm (Figure 5b).75 In addition to intense CH=N signal at 8.15 ppm, several low intensity signals appeared in the region 8.1 - 8.5 ppm with a concomitant appearance of new signals in the -CHO region at 9.4 - 9.6 ppm, in addition to a singlet -CHO signal at 9.6 ppm in V-fa, Figure 5b and S15. Such


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relative changes in magnetic environment can be attributed to the significant interactions of these centres with neighbouring functionalities and their involvement in formation of extensive hydrogen bonding network.76 The variation in intensity of signal due to O-CH2-N, Ar-CH2-N and % ROP is shown in Figure 5d. A time dependent increase in percentage of ROP with consequent decrease in O-CH2-N signal was observed. The % ROP saturates at the 5th day of the reaction indicating a near completion of ROP reaction. The percentage formation of Schiff base was found to be slow as compared to % ROP reaction, which is attributed to the reversibility of the reaction in solvent and slower rate in attainment of equilibrium.77 The presence of Schiff base carbon was also confirmed by the 13C-NMR spectrum of reaction mixture where appearance of signals in the region 162 - 168 ppm, Figure 5c (assigned in accordance to Vf- and Va-Schiff azomethine carbon, Figure S7b, 8b), corroborates well with the 1H-NMR spectrum, Figure 5b. Furthermore, the signal at 194 ppm due to -CHO carbon confirms the presence of residual V-fa and an incomplete conversion to Schiff base which is in good agreement with FTIR results (Figure 3a).




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Figure 5. Stacked time dependent normalized 1H-NMR spectra of C:V (1:2.3) at different time intervals at 70 oC with D2O signal as reference: a) 2.5 - 4.7 ppm region; b) 8 - 9.8 ppm region; c) 13C-NMR of C:V (1:2.3) after 7 days showing formation of Schiff base and absence of OCH2-N signal. Inset shows the presence of O-CH2-N signal at 30 min under similar reaction conditions; d) Time dependent monitoring of 1H-NMR of C:V (1:2.3). A variation in the intensity ratio of the N-CH2-Ar and O-CH2-N signals with respect to the -OMe signal, and percentage change in ring-opening reaction and Schiff base formation. NMR kinetics of V-fa was also monitored without CS, to understand the structural changes associated in presence of CS mediated ROP. 1H- and 13C-NMR spectra of V-fa (Figure S15, S16) showed relatively much slower decrease in intensity of O-CH2-N signal with the retention of it up to 8 days. The prominence of N, O-acetal linkages signals in 4.35 - 4.40 ppm is evident which are inconspicuous in CV2.3. The signal corresponding to ~8.15 ppm is not observed reconfirming the azomethine hydrogen in CV2.3. With time, an extra signal in aldehyde region appeared but comparatively lesser to the reaction mixture suggesting formation of similar type of ring-opened structures without substantial variation in magnetic environment. Surprisingly, % ROP obtained was meagre despite effective decrease in O-CH2N signal suggesting minimal oligomerization (Figure S17). However, % ROP is very high in presence of CS which is attributed to the trapping of more V-fa molecules enhancing their localized concentration in high viscosity CS matrix as a host-guest association, preventing their escape and encountering each other with favourable energy to form polymer. To understand 15 ACS Paragon Plus Environment


what happens, if the reaction is conducted in absence of aldehyde group, structural analogue to V-fa (G-fa) was also chosen for NMR kinetic studies. We found the lower percentage of ROP in G-fa (~50% vs V-fa ~71%), inferring the aldehyde group in V-fa is playing the additional role to assist ring-opening reaction (Figure S18a-d). XRD analysis. XRD pattern of pure CS film showed a lower intensity broad peaks at 2 ≈ 8.2, 11.4, 18.2 and 23o which corresponds to d-spacing of 20.8, 7.5, 4.9, and 3.9 Å respectively,78, 79

Figure 6. The former two peaks are associated with the crystalline form II of CS due to H-

bonded network with appreciable deacetylation degree.75 V-fa monomer showed sharper diffraction peaks due to its non-polymeric nature. In comparison to CS, XRD of CS/V-fa copolymers exhibited neither features of V-fa nor CS confirming the change in phase and structural interactions. In copolymers, a weak broad band at 2 ≈ 2.8o and 6o suggested occurrence of V-fa/PV-fa linked layer in between CS chains. This accounted to the disturbance in H-bonding network of CS in an irregular fashion as evident from disappearance of CS peaks in small angle region. The non-periodic incorporation of V-fa units (at a lower ratio) in CS backbones enhanced the inter-chain distance between chains of CS remarkably, in a similar fashion as reported in earlier reports on CS based host guest structures.75 A very weak broad band in the region 11 - 28o is noticeable in all the copolymers. This can be accounted to the formation of PBz as a result of ROP reaction33 and may also be responsible for disruption of packing of CS chains accounting to the formation of amorphous domains during copolymerization. In comparison to copolymers with lower V-fa ratios, this broad peak is more prominent in CV1 due to formation of more PV-fa rich domains as a result of higher content of V-fa. Zin et al.80 and Barboiu et al.81 reported 2 peak centered at 21o corresponds to a dspacing of 4.4 Å which is further favourable for  -  stacking interactions which may support formation of self-assembled PV-fa chains in similar fashion to some extent.


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Figure 6. XRD patterns of V-fa monomer (as powder), and films of CS and CV copolymers at room temperature. Probable structural organization of ring-opened V-fa/PV-fa units across CS backbone. SEM analysis. SEM micrographs of copolymers revealed a significant effect of V-fa incorporation in CS. The morphology of film surfaces and extent of distribution at different loadings of V-fa in CS was investigated by SEM, Figure 7. There is an obvious difference between the surfaces of neat CS and copolymer films. The solvent evaporation rate, solubility parameters and the polymer/solvent interactions are also known to affect the surface morphologies. To minimize these variations, we have prepared all the samples under similar conditions. The extensive hydrogen bonding in CS film is contributing to a relatively ordered arrangement of the attached CS chains showing an even morphology without features of surface roughness. Covalent interactions via ring-opening reaction, imine bond formation and extension of physical interactions via H-bonding and  -  stacking of PBz in between CS backbone resulted in a remarkable modification of the morphology of films. The smoother surface of CS became morphological inhomogeneous with a significant enhancement of roughness with increase in the extent of substitution at CS by V-fa which disturbs the Hbonding network of crystalline CS in a random fashion. The exfoliation of sheets is accounted to the random arrangement of V-fa linked to CS at lower loadings due to inter- and intramolecular hydrogen bonding along with covalent reactions.82 However at higher loadings, CV0.7 - CV1, showed a regain in relatively ordered arrangement due to considerable interplanar placement of self-organization of PV-fa chains in CS, disrupting the initial intramolecular H-bonds with development of an array of inherent characteristics of intermolecular H-bonding and covalent interactions, in accordance to XRD and FTIR studies. 17 ACS Paragon Plus Environment


Figure 7. SEM micrographs of films of CV copolymers at different resolution a) 100 m; b) 4 m. The samples were prepared under the similar conditions. Based on the FTIR, NMR and XRD analysis, the probable favourable interactions and structural modifications in CS/V-fa copolymers can be represented as in Scheme 2.

Scheme 2. Mechanistic representation of interaction of V-fa with CS showing formation of covalent and physical linkages. Thermal analysis. DSC and TGA analysis has been carried out to investigate thermal behaviour of CS, V-fa and copolymers. Benzoxazine undergoes thermally ring-opening polymerization to form PBz as monitored by DSC studies, Figure 8 and Table S1. Pristine V18 ACS Paragon Plus Environment

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fa showed a melting transition at 124 oC followed by ROP with Ti of 179 oC with polymerization enthalpy of 35.5 kJ/mol. CS showed an endothermic transition at 80 oC due to loss of water with a Tg of 178 oC.83 In copolymers up to CV1 composition, the endotherm and exotherm transition in V-fa due to melting and ROP vanished completely. This reconfirms completion of ring-opening reaction of V-fa by CS, which is in congruence with both FTIR and NMR studies. Previous reported work also supported the above observations, where To of BA-a monomer was lowered down by 27 - 30 oC in the presence of CS (2-10%).84 DSC trace, Figure 8b, of a physical blend C:V (1:1) of same composition as CV1 (chemically reacted) showed respective transitions due to V-fa and CS confirming CS assisted ROP in V-fa monomer in all the CVx copolymer studied, and that too, at very low temperature. Moreover, a further increase in V-fa content in copolymers i.e. CV2 (beyond CV1) showed a residual unpolymerized V-fa content, as visualized from the DSC trace, Figure 8c. Additionally, this suggested inefficiency of CS to open up benzoxazine ring of V-fa at higher loadings. The lowering in curing enthalpy due to ROP to 11.6 kJ/mol in CV2 (Figure 8c) than pristine V-fa (Figure 8a) is confirmation of incomplete polymerization of Bz monomer and presence of residual V-fa, corroborates well with the FTIR studies (Figure S9). The residual V-fa in CV2 showed a bimodal melting peak at 120 and 125 oC signifying differential packing of V-fa units.

Figure 8. DSC traces: a) CS, neat V-fa and CV copolymers; b) physical blend of C:V (1:1), prepared as a refernce for CV1 and c) CV2. Chitosan and CV copolymers are hygroscopic in nature so processing and storage conditions of the films were specifically maintained to minimize the errors in determination of glass transition temperature (Tg) by DSC. A wide range of Tg values of neat CS were reported in literature suggesting a significant contribution of molecular weight and degree of deacetylation.85 All the copolymer films (CV0.1 to CV1) showed a single Tg value, confirming 19 ACS Paragon Plus Environment


a good miscibility of V-fa linked chitosan due to favourable compatible interactions between the CS-polybenzoxazine network.85 Tg for pristine CS was determined as 178 oC and a simultaneous decrease in Tg was observed with increase in V-fa content in the copolymer (Figure S19). The reported Tg of the poly(V-fa) is 270 °C.64 It is expected that the copolymerization of V-fa with CS led to the improvement in Tg of the copolymer. However, the lowering in Tg, in our case, may be attributed to the the possibility of development of hybrid networks with variable physical and chemical interactions. An increase in V-fa content in the blend ratio, subsequently account to an enhanced in situ content of water molecules. In CS blends, presence/incorporation of small molecules including water is known to lower Tg due to induced plasticization effect.86 Furthermore, internal plasticization is also associated due to formation of flexible linkages via attachment of dangling V-fa/branching in CS backbone leading to the disruption of packing of CS chains. This led to an enhancement in rotational degrees of freedom thereby introducing more flexibility in the polymer chain with subsequent lowering in Tg.87 TGA and DTG traces of copolymers are shown in Figure 9a-b and results are summarized in Table S1. Pristine CS showed a three-step mass loss behaviour. The initial mass loss < 150 oC is attributed to the loss of water adsorbed in films due to the presence of hydrophilic functionalities which corroborates well with DSC data. The mass loss at ~300 and ~680 oC is accounted to chain degradation of CS. CV copolymers revealed a similar thermal degradation pattern. As compared to pristine CS, copolymers exhibited a higher thermal stability and which is found to increase with an increase in Bz content. The copolymer composition up to CV0.1 showed a lower thermal stability accounting to initial disruption of more thermally stable Hbonding network in CS due to newly developed small scattered PV-fa regions in CS, as supported by XRD. However, CS to CV2 showed both higher T10% ( = 111 oC) and char yield ( = 28%) leading to substantial improvement in thermal stability. Furthermore, the char yields were found to be in the range of 9.5 to 37.1%, which increase with increasing Bz content. This ~ 3-fold enhancement in char yield in copolymers is due to increase in PV-fa content. CV1 and CV2 prepared under similar conditions showed similar thermal stability despite presence of unreacted V-fa in latter suggesting 1:1 blend ratio is an optimal ratio for ring-opening reaction. To understand whether the residual V-fa in CV2 has any effect on thermal stability. The CV2 blend was heated at 180 oC for 1 h. The thermally treated film CV2 (180 oC) revealed an enhanced thermal stability by 100 oC than CV2 which is attributed to both loss of adsorbed water and polymerization of residual V-fa monomer to a thermally stable 20 ACS Paragon Plus Environment

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crosslinked PV-fa, Figure 9c. Inter-planar placement of PV-fa units in CS accounted to enhanced thermal stability due to incorporation of thermally stable PV-fa and augmentation of crosslinking density via extended physical and covalent interactions.

Figure 9. Thermal stability of CS, V-fa and CV copolymers: a) TGA thermograms; b) DTG traces; c) effect of higher temperature treatment (180 oC) to CV2. Tensile testing of CS based films. The effect of incorporation of Bz monomer on the tensile properties of CS/V-fa films was studied, Figure 10. Stress-strain curves of thin films of neat CS and copolymers was determined without any further thermal treatment. Mechanical properties of CS films are strongly affected by the average molecular weight of neat CS and the preparation conditions. With increase in PV-fa content, the % elongation reduced suggesting enhancement in crosslinking density due to ring-opened structure of V-fa in CS which imparted brittle nature to the films. Neat CS film displayed an average tensile strength of 18.2 MPa and modulus of 400 MPa. The tensile strength and modulus of the CS/V-fa copolymers were significantly enhanced with the addition of V-fa, Figure 10b. The tensile strength of CS film increased by 1.7-fold while modulus by ~ 5-fold at 50% loading of V-fa in CV1. It is worth noting that incorporation of V-fa in CS significantly led to an increase of crosslinking density, morphological changes, amorphous behaviour along with favourable interactions83 accounting to an increase in tensile strength and storage modulus. At CV1 composition, a homogeneous distribution of V-fa monomer and its polymerization led to a uniform matrix of CS and PV-fa further minimizing the stress concentration centres accounted to a highest value amongst the studied ratios of copolymers.



Figure 10. Tensile properties of films of CS and CV copolymers: a) stress vs strain curves; b) tensile strength and modulus.

CONCLUSIONS Sustainable polybenzoxazines have attracted enormous attention in recent years due to the high abundance, low cost and renewable nature. Copolymerization of biobased monomer, vanillin benzoxazine monomer (V-fa) with naturally existing natural biopolymer, CS effectively lowered the ROP temperature to 70 oC (from 179 oC of neat V-fa) which is superior to the most of the reported catalyst. In addition, the processability of V-fa into thin films was achieved enabling further scope of their utility in applications. In situ NMR studies of copolymers provided knowledge on relative changes in structural features and involvement of functionalities in assisting opening of benzoxazine ring of the monomer. The quantitative estimation studies revealed imine-bond formation on chitosan backbones improved significantly in solid state films (determined by FTIR) than in aqueous solutions (determined by NMR) due to covalent dynamic behaviour of reversibly interacting components, aldehyde of V-fa and amine of CS. Further, structure-morphology correlations revealed a progressive incorporation of covalently and physically linked V-fa units onto chitosan backbones which polymerized to form PV-fa matrix in the inter-planar region of CS. Improved thermal and tensile properties of CS are ascribed to both favourable interactions and changes of crystallinity due to V-fa incorporation. Current work demonstrated efficient metal-free ROP reaction and development of hierarchical growth of benzoxazine network in between CS chains. Such hostguest interactions of V-fa and CS with the utility of their inherent functionalities is a promising strategy to extend the platform of their futuristic applications.


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ASSOCIATED CONTENT Supporting Information. Experimental details, viscometry studies, FTIR spectra, reaction 1H-,

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An effective and simple strategy for preparation of low temperature curable ecofriendly polybenzoxazine-chitosan biopolymers with enhanced thermal and mechanical properties.


Sustainable Framework of ChitosanâBenzoxazine with Mutual

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